FIGS. 1A and 1B show top and side views, respectively, of a prior art reactor, configured as a cylindrical reformer 100. The cylindrical reformer 100 includes a cylindrical compartment 101 forming a reaction vessel. The reformer 100 comprises one or more pulse heaters 102A, 102B, each of which comprises a pulse combustor 104A, 104B connected to a respective resonance tube 106A, 106B. As seen in FIG. 1A, the pulse heaters 102A, 102B extend in one direction across the diameter of the cylinder. Air and fuel products enter the pulse combustors 104A, 104B and the combustion products or flue gas exit the resonance tubes 106A, 106B.
The pulse heaters 102A, 102B are of the sort disclosed in U.S. Pat. No. 5,059,404, whose contents are incorporated by reference to the extent necessary to understand the present invention. Such pulse heaters are configured to indirectly heat fluids and solids introduced into a reformer reaction vessel 101. The resonance tubes 106A, 106B associated with the pulse heaters 102A, 102B serve as heating conduits for indirectly heating contents of the compartment 101.
As seen in FIGS. 1A and 1B, a second pair of pulse heaters 108A, 108B are directed at right angles to the first pair of pulse heaters 102A, 102B across the diameter of the compartment. As seen in FIG. 1B, this leaves vertically extending quadrants 136 within the compartment 101 in regions defined by the crossing pulse heaters.
The pulse heaters are immersed in a dense fluid bed 110, which extends from the compartment bottom 112 to approximately the top bed line 114. The bottommost pulse heater 102B is located at a height H1 meters above the distributor 122 to avoid painting the resonance tubes 104B with liquor 118. In some prior art systems, the height H1 is about 2-3 meters.
Spent liquor 118 is injected into the side of the compartment 101 near the bottom of the dense fluid bed 110. Generally speaking, the spent liquor is introduced into the compartment via a plurality of inlets 103 that are circumferentially arranged around the cylindrical compartment 101. Though in FIG. 1B, only four such inlets 103 are shown, it is understood that other numbers of circumferentially arranged inlets may be provided. In other prior art embodiments, the spent liquor may be introduced through the bottom of the compartment 101 through a plurality of inlets more or less evenly distributed across the bottom, perhaps arranged in an array or other pattern.
Superheated steam 120, or other fluidization medium, enters from the bottom of the compartment 101 and passes through a distributor 122. The distributor 122 helps uniformly spread the entering steam 120, which then percolates through the dense fluid bed 110. Product gas 124 leaves from a freeboard area 126 at the top of the compartment 101 after passing through one or more internal cyclones (not shown) used to help drop out entrained bed solids.
FIGS. 2A and 2B show an alternative prior art configuration in the form of a rectangular reformer 200. The rectangular reformer 200 has a compartment 201 with a rectangular cross-section as seen from above (See FIG. 2B). A plurality of pulse heaters 102 arranged in one or more rows pass through this compartment 201. The rows are staggered relative to each other to enhance heat transfer. Each of these pulse heaters 102 comprises a heating conduit in the form of a resonance tube for indirectly heating the contents of the compartment 201.
A distributor 222 is provided at the bottom of the compartment 201, much like in the cylindrical reformer 100. The bottommost pulse heaters 202 are located at a height H2 above the distributor 222. In some prior art systems, this height H2 is again about 2-3 meters. Moreover, just as in the case with the cylindrical reformer, spent liquor 218 is introduced into the side of the compartment 201 near its bottom. Generally speaking, the spent liquor is introduced into the compartment via a plurality of inlets 203 that are arranged along the walls around the rectangular compartment 201. In other prior art embodiments, the spent liquor may be introduced through the bottom of the compartment 201 through a plurality of inlets more or less evenly distributed across the bottom, perhaps arranged in an array or other pattern. Meanwhile, product gas 224 leaves from a freeboard area 226 at the top of the compartment 201. It is understood that the operation of the rectangular reformer 200 is similar to that of the cylindrical reformer 100 described above, in most material respects.
Upon injection into the fluid bed 110, the carbonaceous feedstock undergoes drying, devolatilization, char formation and char conversion. In a steam reforming environment, all of these processes are endothermic i.e. require heat input. An issue in the prior art configuration is that drying, devolatilization, char formation and char conversion processes all compete for heat transfer and mass transfer in the region that is above the distributor but below the bottom pulse heater. All these processes are heat sinks and the entering fluidization medium 120 may be another heat sink if it is steam and is at a temperature below that of the fluid bed. The only heat sources are the pulse heaters and these are significantly removed from the heat sinks by the aforementioned distances H1 and H2 in the prior art embodiments described above. The only link is the solids circulation rate and if this is not up to par, the feedstock injection region starves for heat and the reactor performance suffers.
In addition, both heat transfer and mass transfer are important for satisfactory char conversion. The higher the char temperature and the reactant or steam concentration, the greater the char conversion rate. The region just above the distributor 122, 222 is characterized by high steam or reactant concentration, which is favorable for char conversion, provided the char temperature could be maintained at the fluid bed temperature. Due to feedstock injection and reduced solids circulation rate, the heat supply is limited which is likely to depress the char temperature and in turn the char conversion rate. In the region of the pulse heaters, the heat transfer is good but the mass transfer may be unsatisfactory if the reactant (steam) bypasses due to channeling, again impairing char conversion.
Commercial units generally require deep or tall dense fluidized beds to accommodate the large number of heat transfer tubes. Operating these units in bubbling fluidization regime is rather limiting from heat and mass transfer and gas/solid contact standpoints due to the relatively large bubbles, increased bubble coalescence and the propensity for steam/gas bypassing. Conversely, operation in the turbulent fluidization regime affords good gas/solid contact and excellent heat and mass transfer characteristics. This, however, requires a significantly higher superficial fluidization velocity than that for the bubbling regime. One feasible approach is to select a different heat exchanger configuration and a smaller bed material mean particle size.